The invention relates to the transfer of IP (Internet Protocol) data in a telecommunications system and particularly in a system providing compression of IP data header fields.
Rapidly advancing IP technology has expanded the field of use of diverse IP-based applications beyond the conventional Internet data transfer. IP-based telephone applications in particular have developed rapidly, due to which an ever increasing portion of the transfer path of the calls can be implemented using IP technology. Mobile communications networks in particular form an area where IP technology is anticipated to provide a host of advantages, because in addition to conventional speech services, which could be provided using different IP-based voice applications, mobile communications networks will be increasingly offering diverse data services, such as browsing of the Internet and electronic mail services, which are typically most advantageously produced as packet-switched IP-based services. Hence, IP-layers adapted to the protocols of the mobile communications system could be used for providing both audio/video services and various data services.
In mobile communications systems it is particularly important that the limited radio resources can be used as efficiently as possible. This, on the other hand, complicates the use of IP protocols on the radio interface because in IP-based protocols the proportion of different header fields in the data to be transferred is large and, consequently, the proportion left for the payload is small. Due to the limited radio resources, this ratio needs to be made smaller. For this purpose, header field compression methods, such as ROHC (Robust Header Compression) of IETF (Internet Engineering Task Force), have been designed. In this application, the term ‘payload’ is used for data which is substantially useful for the application employed and ‘header fields’ for fields added into the payload by lower layers taking care of data transfer of the application. In voice applications, the payload comprises for example voice samples and control data, the header fields on the network layer being for example RTP, UDP and IP header fields.
The proposed compression methods require uncompressed header fields to be transmitted at the beginning of a connection and possibly in a periodic manner. ROHC makes use of a plural number of compress states, the efficiency of the compression increasing as the operation proceeds to a higher state. A basic principle is that the compression is always carried out in the highest state possible, provided, however, that the compressor has sufficient certainty of the decompressor having enough information for carrying out decompression in the same state.
A logical connection is typically allocated to a convergence entity providing transmission of the application layer data flow to the mobile communications network on one hand and to the convergence entity of RNC on the other, the logical connection being used for transferring IP packets to the physical layer. According to the standards of the third generation mobile communications system UMTS (Universal Mobile Telecommunications System) the packet data convergence protocol (PDCP) layer entity always uses one radio link control (RLC) layer connection for transferring a data flow. When the RLC connection, and thereby the logical connection, is being allocated, parameters determining the characteristics of the logical connection, such as the quality of service level of the connection, are negotiated.
In the transmission of Voice over IP (VoIP) in particular, header fields may require considerably more bits than payload. Some of the header fields to be transferred may be compressed, which is why the size of the header fields in the IP packets to be transferred may vary considerably. Since the payload of the IP packets and the header fields compressed in different ways are transferred on the same logical connection conforming to the negotiated parameters, optimal use of radio resources is not achieved in the data transmission. A large amount of capacity must be reserved especially for an IP packet flow generated by a wideband speech codec WB AMR (Wideband Adaptive Multirate Codec) designed for UMTS, which leads to uneconomical use of the code tree.